Pulse amplitude modulation (PAM) is a form of signal modulation where bit values are encoded in the amplitude of signal pulses. PAM4 is a form of PAM in which four distinct amplitude levels are used, each amplitude level encoding a different combination of two bits. PAM4 signaling has shown promise for high speed signaling (e.g., greater than 56 Gbps), compared to non-return to zero (NRZ) signaling.
A challenge of PAM4 signaling, compared to NRZ signaling, is that in PAM4 signaling the signal swing is decreased by ⅓ (or 9.5 dB) compared to NRZ signaling. This is even more problematic when considering technologies that require low power supplies and low breakdown voltages. In addition, bandwidth limitation at high data rates (e.g., greater than 56 Gbps) has been found to degrade the vertical eye opening in the eye diagram.
There have been attempts to increase the output swing using a high efficiency voltage mode driver in combination with a current mode logic (CML) current boost circuit. However, the increase to voltage swing achieved in these attempts may not be sufficient, particularly at high frequencies (e.g., greater than 56 Gbps). A finite impulse response (FIR) filter may be used to provide some voltage boost, however this requires significant power and physical area to implement. The use of a FIR filter may also be limited by the number of taps that must be implemented.